The present invention relates to a semiconductor device and concerns a technique applicable to a semiconductor device having a transistor formed in a compound semiconductor layer.
Since compound semiconductors such as GaN have a band gap larger than that of silicon and have a larger critical electric field, they can easily form high voltage transistors with low loss. However, it is difficult to obtain p-type compound semiconductors. Accordingly, it is difficult to form p/n junction diodes as protective devices for the transistors.
As a countermeasure, Japanese Unexamined Patent Publication No. 2007-59589 describes that a Schottky barrier diode is used as a protective device for a transistor. In Japanese Unexamined Patent Publication No. 2007-59589, a Schottky electrode is formed over the surface of a GaN layer in the Schottky junction of the Schottky barrier diode.
Japanese Unexamined Patent Publication No. 2001-210657 describes that a depletion type transistor and an enhancement type transistor are arranged over one identical substrate. In Japanese Unexamined Patent Publication No. 2001-210657, a substrate has a structure comprising a p-type GaN layer, a n-type GaN layer, and an AlGaN layer stacked in this order. Then, in the n-type GaN layer and, the AlGaN layer, an acceptor (p-type impurity) is introduced into a region situated below the gate electrode of the enhancement type transistor.